Facebook games are big business for some developers, so much so that their own currency has real value that can be used to further drive engagement within their games.

The social network has allowed developers to insert “Payer Promotion” into their apps through a DealSpot unit, but today has released a new API for further customization.

With this new API, Facebook developers can now determine who is eligible for a promotion and then show them a customized unit to drive engagement. Here’s what the company had to say:

Creating a custom unit allows you to show the offer at the right time and with the right context to increase conversion. For example, if a player needs to upgrade his or her command center before going into battle, you can show the offer and provide an extra incentive for a free upgrade, further encouraging that player to accept. KIXEYE created a custom unit for the promotion and has grown payer conversion 18% overall and seen a 8% lift in revenue in War Commander since implementing the feature.

In-app promotions are pretty hot right now, as companies like Kiip are using “rewards” as ad-units to drive brand affinity and relationships between developers and their users.

A few months ago, Kiip’s CEO Brian Wong told us that his team was working on HTML5 units that could be placed within Facebook games, so it’ll be interesting to see if that will come to fruition to beat Facebook at its own…game.